1. Field of the Invention
The present invention is directed to a method of controlling two or more turbocompressors each having a control device for maintaining a distance of an operating point relative to the surge limit curve of the respective turbocompressor and to a system and a computer readable medium for implementing said method.
2. Description of the Related Art
The performance map of a turbocompressor is divided by the surge limit curve into a stable region and an unstable region. Surging occurs when the operating point of the turbocompressor enters the unstable region of the performance map due to a reduction in the flow rate or an increase in the pressure ratio. This results in cyclical delivery and backflow of the compressed medium, often accompanied by high vibrations, pressure surges, and a rapid rise in temperature in the turbocompressor. Possible consequences are, for example, bearing damage, rubbing damage, and damage to the impeller or impeller vanes, which could lead to operating failure.
In order to prevent this, control devices are known for maintaining a fixed or predefinable distance of the operating point from the surge limit curve and controlling the operating point of the turbocompressor, for example, correspondingly changing its speed or opening a relief valve. Hereinafter, regulation, i.e., a control based on fed-back actual values and comparison thereof with reference values, will also be referred to as control.
If a plurality of turbocompressors are connected together in such a way that they jointly convey a medium, a control device is generally provided for each compressor to maintain its distance from the surge limit curve because prevention of surging should be of primary importance.
Without any coordination of these control devices, an uneven load sharing may come about between the individual turbocompressors, and the entire system can become unstable due to control devices acting counter to one another.
Therefore, it is known to provide a load sharing control by means of which the operating points of the individual turbocompressors are selected in such a way that all of the compressors have substantially the same distance from their respective surge limit curve. This load sharing control works without central or master controllers and is based on intercommunicating control devices of the individual compressors.
In contrast, U.S. Pat. No. 6,602,057 B2 discloses a central control which carries out the load sharing based on flow capacity coefficients of the individual compressors. For load sharing of interconnected compressors, U.S. Pat. No. 5,743,714 discloses minimizing the total work. In U.S. Pat. No. 6,233,954 B1, a high-ranking compressor is selected and the operating points of the individual compressors are then set depending on this high-ranking compressor. These concepts do not take into account the distance from the surge limit curve in load sharing.
In the above-mentioned load sharing control, a manual trimming or adjustment of the individual control devices is provided, by means of which the distance to be maintained from the surge limit curve can be changed relative to a distance that is substantially identical for all of the compressors. By trimming is meant herein a displacement of that distance of an operating point from the surge limit curve that is sought to be maintained preferably by a control device in a turbocompressor. Accordingly, trimming of this kind can shift the operating points of the individual turbocompressors, i.e., the load sharing control can be manually overdriven to a certain extent, for example, to deliberately load or unload individual compressors to different degrees or to operate them closer to or farther away from their surge limit.
However, while the known load sharing control selects substantially the same distance from the respective surge limit curve for all of the turbocompressors in such a way that the interconnected compressors, as a totality, present the desired total process variables, for example, a total flow capacity and a total pressure ratio, manual trimming of individual control devices without taking into account the rest of the turbocompressors leads to an unwanted change in these total process variables. For example, if the distance from the surge limit of only one compressor of a group of two turbocompressors is reduced, this leads to a reduction in its flow capacity with the pressure ratio remaining the same and, therefore, also to an unwanted reduction in the total flow capacity of the group.
In order to maintain a total process variable, U.S. Pat. No. 5,347,467 proposes a central controller which jointly adjusts local anti-surge controllers. Specific trimming of individual control devices is not provided.